The present invention relates to a bias magnetic field impress apparatus using a permanent magnet for an opto-magnetic disk apparatus.
Optical disk apparatus records information by magnetizing a magnetic layer of an opto-magnetic disk, in the vertical direction. When in the recording or the erase modes, a bias magnetic field impress apparatus becomes necessary to impress a bias magnetic field in the vertical direction with respect to the portion of the disk to which a laser beam is to be irradiated.
Permanent magnets have been used as a bias magnetic field impress apparatus.
Bias magnetic field impress apparatus using these permanent magnets have a permanent magnet which rotates so as to correspond to mode changes in the opto-magnetic disk apparatus. When in the recording or the erase mode, the permanent magnet is stopped at a position where the magnetic field has to operate in the vertical direction with respect to that portion of the disk to which the laser beam is to be irradiated, and when in the playback mode, the permanent magnet is stopped at a position where the magnetic field does not act in the vertical direction with respect to portion of the disk to which the laser beam is to be irradiated.
(1) It is desirable that these bias magnetic field impress apparatus be able to impress a favorable bias magnetic field when there is the recording mode and the erase mode, and without the generation of any unnecessary field, and (2) it is desirable that the configuration be as compact as possible and with as few parts as possible.
In conventional bias magnetic field impress apparatus, two permanent magnets are disposed at both sides of a rectangular prism-shaped permanent magnet for impressing the bias magnetic field, which two permanent magnetic are used to hold the permanent magnet described above at a position parallel to the disk.
The magnetic field of these permanent magnets used for holding the permanent magnet tended to influence the bias magnetic field impressed to the opto-magnetic disk by the permanent magnet described above, for impressing the bias magnetic field.
In addition, the provision of permanent magnets for holding increased the number of parts in the bias magnetic field impress apparatus and made it difficult to make them more compact.
FIG. 1A indicates the state for the recording or the erase mode of a conventional bias magnetic field impress apparatus. FIG. 1B indicates the state for the playback mode.
In the figures, a permanent magnet 10 is for impressing a bias magnetic field, and is of a shape of rectangular prism, with magnetic poles on both sides in the direction of the width. This permanent magnet 10 is fixed on a rotating shaft of a stepping motor 11 and is disposed so that it confronts a disk 12 at a surface opposite to the other surface opposing an objective 13, and extends in the direction of the radius of the disk 12.
In addition, on both sides of the permanent magnet 10 are provided permanent magnets 14 and 15 to hold the permanent magnet 10 in a position where it is parallel to the disk 12.
When in the erase and the record modes, the permanent magnet 10 is rotated by the stepping motor 11, to a position where it is vertical with respect to the disk 12, as indicated in FIG. 1A, and is held in that position by the stepping motor 11, and the magnetic field is impressed in the vertical direction to the disk 12.
When there are modes other than the erase and the recording mode, the holding current to the stepping motor 11 is interrupted and the permanent magnet 10 is magnetically pulled back by the permanent magnets 14 and 15 to be held in the state indicated in FIG. 1B.
The permanent magnets 14 and 15 for holding, are provided in the vicinity of the permanent magnet 10 and the disk 12 and so the bias magnetic field impressed to the disk 12 by means of the permanent magnet 10 receives the influence, and this causes recording and erase to be adversely affected.
In addition, through the provision of the permanent magnets 14 and 15 for holding, the number of parts in the bias magnetic field impress apparatus increases and it is difficult to make the apparatus more compact.